The European Patent Application No. 0 114 085 published on July 25, 1984 discloses a dimensionally stable anode for an aluminum production cell comprising a conductive substrate of a ceramic, a metal or other materials which is coated with a layer of cerium oxycompound. The anode is essentially stable under conditions found in an aluminum production cell, provided that a sufficient content of cerium is maintained in the electrolyte.
The anode as described in the above European patent application performs well in respect of dimensional stability, however, contamination of the produced aluminum by substrate components may occur under certain circumstances. As shown by microphotographs, the cerium containing coating can be comprised of a non-homogeneous and non-continuous structure leaving small interstices between coated areas, which provide access of the electrolyte to the substrate. In such cases, the electrolyte may corrode the substrate leading to a limited but undesired contamination of the aluminum by substrate components.
It had also been speculated that the above described coating may consist of other rare earth metals such as praseodymium, samarium, europium, terbium, thulium or ytterbium in a suitable concentration. However these elements are not easy to be coated under the conditions provided in the above publication which does not contain any instructions as to how these elements may be coated onto the substrate, nor in which ranges of concentration. Further, it does not contain any suggestion as to a possible beneficial effect of these elements.
The French patent application No. 2 407 277 discloses a method of electrolyzing chlorides of e.g. magnesium, sodium, calcium or aluminum in electrolytes having temperatures between 500.degree.-800.degree. C. using an anode comprising a substrate and a coating of an oxide of a noble metal, whereby a certain concentration of an oxide or oxychloride of a metal which is more basic than the metal produced is maintained in the bath. Thus, by increasing the basicity of the bath the solubility of the anode coating is reduced.
This method provides better stability of the anode coating by the addition of melt additives, however, these additions relate to the stabilization of the coating rather than to the improvement of the coating morphology and does therefore not contribute to the improvement of the substrate-protection which is not always completely satisfactory in the case of a pure cerium oxycompound coating being one of the hereunder defined objects of the present invention. The substrate itself which is essentially protected by the coating and only subject to corrosion at finite deficient locations thereof may not simply be protected against corrosion by modifying the basicity of the bath as described in the French patent, since the anode substrate according to the present invention is unstable in a fluoride bath at e.g. 960.degree. C. and needs therefore to be completely shielded therefrom. A mere modification of the basicity would not improve the stability of the substrate as it does with a coating of an oxide of a noble metal which is essentially stable in the bath per se.